In electric camshaft adjusters, the low torque of the electric adjustment motor must be converted into a high torque which is necessary to adjust the camshaft. So-called three shaft gearings (summing gearings) are used for this purpose. Their drive is provided by means of a drive wheel which is fixed to the crankshaft and a drive shaft, while the power take off is carried out by means of an output shaft and a output component which is fixed to the camshaft. The adjustment power is input into the three shaft gearing in one direction or the other by the electric adjustment motor by means of an adjustment shaft.
Stringent requirements are made of the three shaft gearing. They are intended to have a high degree of efficiency so that the electric adjustment motor and its dissipated heat remain small. In addition, the tooth play of the toothings should be kept as small as possible since otherwise the highly varying alternating torque of the camshaft leads to undesired noises. This is all the more the case since the tooth edge play between the toothings of the adjustment shaft, drive shaft and output shaft is increased by the transmission ratio, thus promoting the generation of noise.
Furthermore it is necessary to minimize the installation space for the three shaft gearings since modern vehicle engines have to be made increasingly compact in order to comply with the safety-related minimum distance between the bodywork and the vehicle engine.
Finally, the three shaft gearing must be capable of being manufactured cost-effectively in order to keep the system costs for an electric camshaft adjuster with an adjustment gearing, adjustment motor and electronic actuation system low.
Possible three shaft gearings are double planetary gear mechanisms, double and single eccentric gear mechanisms and harmonic drives. In particular, the latter seem to be suitable for fulfilling the above requirements. In this context, two versions of the harmonic drive are known, specifically the harmonic drive of the pot design and that of the sleeve design.
EP 1 039 100 A2 and EP 1 039 101 A2 present harmonic drives of the pot design. This design requires a relatively large amount of axial installation space and due to the principle has axial thrust which requires corresponding bearings. Furthermore, with the pot design there is the risk of the wave generator tilting and jamming as a result of the toothing of the sleeve, especially since floating bearing thereof is virtually impossible because of its one-sided screw connection. The toothing of this design requires a special section which makes shaping manufacture difficult. Furthermore, instead of a cost-effective standard grooved ballbearing a costly thin ring bearing is necessary.
WO 95/00748 discloses a camshaft adjuster with a harmonic drive of a sleeve design. In this design there is no axial thrust since the tilting forces in the toothing of the sleeve compensate one another owing to the floating bearing of the wave generator and protect the wave generator against tilting and jamming. Furthermore, the sleeve design takes up comparatively little axial installation space. However, costly thin ring bearings are also used for the wave generator in the example above. In addition, the arrangement of the drive wheel, output component and ring gears requires considerable installation space in the above example.